Interesting… so are you saying that you are using a private i-cal URL in Google calendar? would that not make it publicly accessible after all? It sounds weird. Why would you put a private link on a public site? What happens when a stranger uses the public Google Cal-URL?
Secondly, yes, unfortunately Googles approach is that all services are central to THEM, not a third party. What you really want is a way to directly edit and publish calendar data somewhere (source data). The fact that Apple allows true private (group) calendars is a great work around, but ideally you would want a personal web interface for your client to edit the (any) calendar.
Have you checked out the Event Calendar stack? It has the ability to read a(ny) file to populate the calendar/event list) All you need is a way to generate or update the file.
If any of the available CMS solutions would have a stack that allows for editing a calendar file or a file that is formatted to populate a calendar, the whole google issue and all related complications wil go away. Not sure why no one is jumping all over that…